NOTE: visit https://www.github.com/kreivalabs for the current code version.
On a recent job I had to calculate acoustic velocity to set speaker delays without access to a true RTA. I’m not a big fan of the one-millisecond-per-foot “rule” for anything other than quick mental calculations independent of production conditions. I would rather be more accurate. So, utilizing the following equation to calculate acoustic velocity based on measured temperature in degrees Celsius:
Acoustic Velocity (m/s) = 331.4m/s + (0.6 * temperature Celsius)
I use a digital temperature meter to get degrees Fahrenheit (a better measurement for calculations in the States) and convert as follows:
(Degrees Fahrenheit – 32) * 5/9 = Degrees Celsius
Plug the Celsius value into the equation above to return acoustic velocity in m/s based on measured temperature (because as we know, air temperature determines acoustic velocity).
Then, put that measurement into something more useful, like ft/ms, which is what a standard speaker processor will be looking for.
Take your m/s value and multiply by 0.00328084
Once you know the distance in feet between your zero reference and your delay speaker, multiply that by the ft/ms value above to get your ms offset for the delay. Of course, this will not be perfectly accurate as it can’t take into account air density, just temperature. For indoor use, it’s still pretty good and certainly better than one-millisecond-per-foot.
To make the whole process a bit easier, I wrote some AppleScript that prompts for a temperature value, then returns acoustic velocity in m/s and ft/ms.
Source available at https://github.com/kreivalabs/acousticVelocityAndDelay-AppleScript
# version 1.2 / 08-November-2017 # AppleScript (* A utility to calculate approximate acoustic velocity in air based on measured temperature in degrees Fahrenheit, as well as approximate speaker delay timings in milliseconds based on the measured distance between loudspeaker systems. Created to simplify the process of setting speaker delay timings for indoor performances of theater and small to mid-sized concerts in the absence of an RTA software package. Note: these calculation cannot take into account air pressure or gas molecular density. The returned results are sufficient for most indoor measurements. *) try display dialog "Enter temperature in degrees Fahrenheit" default answer "" with title "Acoustic Velocity Calculator" set myTemp to text returned of result as number display dialog "Enter measured distance between speakers in feet" default answer "" set myDistance to text returned of result as number on error errmsg number errnbr my error_handler(errnbr, errmsg) return end try ignoring numeric strings set myCelcius to (myTemp - 32) * 5 / 9 as number set metersSeconds to (0.606 * myCelcius) + 331.3 as number set feetMiliseconds to metersSeconds * 0.00328084 as number set delayTime to myDistance / feetMiliseconds as number end ignoring display dialog "Approximate Acoustic Velocity is " & ((round (metersSeconds * 100)) / 100) & " m/s, or " & ((round (feetMiliseconds * 100)) / 100) & " ft/ms. Approximate delay time is " & ((round (delayTime * 100)) / 100) & " ms." as text